Vertical resonator-type light-emitting element and vertical resonator-type light-emitting element array

Abstract

The present invention has: a first reflector; a first semiconductor layer formed on the first reflector; an active layer comprising a multi-quantum well layer including first to n-th well layers sequentially formed on the first semiconductor layer, and a final barrier layer formed on the n-th well layer; an electron blocking layer formed on the final barrier layer; a second semiconductor layer formed on the electron blocking layer; and a second reflector formed on the second semiconductor layer. In is included in the content of the first to n-th well layers. The In content in the first to n-th well layers is higher in a well layer located closer to the second semiconductor layer. The EL peak wavelength of the multi-quantum well layer is shorter than the resonance wavelength of a resonator formed by the first reflector and the second reflector. The detuning amount, which is the difference between the EL peak wavelength and the resonance wavelength, is 4 nm or more.

Description

Vertical resonator type light-emitting element and vertical resonator type light-emitting element array

[0001] The present invention relates to vertical resonator type light-emitting elements and vertical resonator type light-emitting element arrays, and more particularly to vertical resonator type light-emitting elements having a multiple quantum well active layer and vertical resonator type light-emitting element arrays in which such vertical resonator type light-emitting elements are arranged.

[0002] Conventionally, vertical cavity surface-emitting lasers (VCSELs) and other vertical cavity-type light-emitting devices are known, which have a structure that resonates light perpendicular to the substrate surface and emits light in a direction perpendicular to the substrate surface.

[0003] In vertical-cavity light-emitting devices, a multiple quantum well (MQW) structure is generally employed in the active layer to obtain low threshold current and highly efficient light emission characteristics.

[0004] For example, Patent Document 1 describes an end-emitting nitride semiconductor laser element having a structure aimed at improving internal quantum efficiency by lowering the electron and hole concentrations in the p-side optical guide layer between the final quantum well layer and the electron barrier layer.

[0005] Furthermore, Patent Documents 2 and 3 describe a mechanism for improving characteristics while considering standing waves in a resonator.

[0006] Furthermore, Non-Patent Documents 1 and 2 describe an effect called CPE (Compositional pulling effect) in crystal growth, and the segregation of InGaN / GaN and AlGaN / AlN, respectively.

[0007] Japanese Patent Publication No. 2014-131019, Japanese Patent Publication No. 2024-131244, Japanese Patent Publication No. 2023-43084

[0008] S. Pereira et al., Physical Review B, vol.64, 205311(2001)B. Liu et al., Applied Physics Letters 98, 261916(2011)

[0009] Conventional vertical-cavity light-emitting devices have faced challenges in further reducing the threshold current and improving luminous efficiency and temperature characteristics.

[0010] The inventors of this invention have found that even when each quantum well layer of a multiple quantum well active layer is formed through a manufacturing process under the same conditions (e.g., growth temperature, growth time, raw material gas phase ratio, etc.), non-uniformity occurs in the peak gain wavelength (band gap) of each quantum well layer. The present invention is based on this finding and aims to provide a vertical resonator type light-emitting element and a vertical resonator type light-emitting element array having excellent high-temperature characteristics and high-power characteristics.

[0011] A vertical resonator light-emitting element according to one embodiment of the present invention comprises: a first reflector; a first semiconductor layer formed on the first reflector; an active layer comprising a multiple quantum well layer including first to n well layers (n is an integer of 3 or more) sequentially formed on the first semiconductor layer and a final barrier layer formed on the nth well layer which is the final well layer of the multiple quantum well layer; an electron barrier layer formed on the final barrier layer; a second semiconductor layer formed on the electron barrier layer; and a second reflector formed on the second semiconductor layer, wherein the first to n well layers have an In composition, the In composition of the first to n well layers is greater for well layers closer to the second semiconductor layer, the EL peak wavelength of the multiple quantum well layer is shorter than the resonance wavelength of the resonator formed by the first reflector and the second reflector, and the detuning amount, which is the difference between the EL peak wavelength and the resonance wavelength, is 4 nm or more.

[0012] Another embodiment of the present invention provides a vertical resonator type light-emitting array having a plurality of the above-described vertical resonator type light-emitting elements, wherein the plurality of vertical resonator type light-emitting elements are separated by grooves extending from the side of the second semiconductor layer to the interior of the substrate and are arranged with the substrate as a common element.

[0013] This is a schematic cross-sectional view showing the structure of a vertical-cavity surface-emitting laser (VCSEL) according to the first embodiment of the present invention. This is a plan view of the vertical-cavity surface-emitting laser as seen from above (top view). This is a schematic plan view of the p-type semiconductor layer and the current injection section as seen from above. This is a diagram schematically showing the band structure of the conduction band of the vertical-cavity surface-emitting laser. This is a flowchart showing the manufacturing method of the vertical-cavity surface-emitting laser. This is a diagram schematically showing the gain profiles of each well layer of the active layer in this embodiment. This is a diagram schematically showing the gain and combined gain of well layers QW1 to QW4 and the EL intensity of the active layer. This is a diagram schematically showing the combined gain profile and EL profile of the active layer at room temperature. This is a diagram schematically showing the gain and combined gain profiles of well layers QW1 to QW4 in the vertical-cavity surface-emitting laser of Comparative Example CX1. This is a diagram showing the L-I characteristics of the vertical-cavity surface-emitting laser of this embodiment. This is a diagram showing the L-I characteristics of the vertical-cavity surface-emitting laser of Comparative Example CX1. This is a schematic cross-sectional view showing the structure of a vertical-cavity surface-emitting laser according to the second embodiment. This is a schematic diagram of the vertical-cavity surface-emitting laser according to the second embodiment as viewed from the top. This is a diagram showing the L-I characteristics of the vertical-cavity surface-emitting laser according to the second embodiment. This is a diagram showing the band structure of the conduction band of the active layer in Modification 1. This is a schematic cross-sectional view showing the structure of a vertical-cavity surface-emitting laser according to Modification 2. This is a schematic cross-sectional view showing the structure of a two-dimensional array type vertical-cavity surface-emitting laser array according to the third embodiment. This is a schematic plan view showing the back surface of the vertical-cavity surface-emitting laser array.

[0014] Preferred embodiments of the present invention will be described below, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially identical or equivalent parts will be denoted by the same reference numerals.

[0015] [First Embodiment] (1) Structure of a Vertical Cavity Surface Emitting Laser Figure 1 is a schematic cross-sectional view showing the structure of a vertical cavity surface emitting laser 10 (VCSEL) according to the first embodiment of the present invention. In this embodiment, the vertical cavity surface emitting laser 10 is a nitride surface emitting laser made of a gallium nitride (GaN) semiconductor layer. Figure 2A is a plan view of the vertical cavity surface emitting laser 10 as seen from above (top view).

[0016] The vertical-cavity surface-emitting laser 10, which is a vertical-cavity light-emitting element, is constructed by sequentially forming the following layers on a substrate 11 in this order: a semiconductor DBR (Distributed Bragg Reflector) 12 (first reflector), a base layer 11B, an n-type semiconductor layer 13 (first semiconductor layer), an active layer 15 consisting of multiple quantum wells, an electron barrier layer 17 (EBL: Electron Blocking Layer), and a p-type semiconductor layer 18 (second semiconductor layer). The active layer 15 is composed of a multiple quantum well layer 15Q and a final barrier layer 15L. In addition, an AR coating 29 (anti-reflective coating) is provided on the back surface of the substrate 11.

[0017] The n-type semiconductor layer 13 has a rectangular columnar shape at its outer edge. The n-type semiconductor layer 13 also has a cylindrical protrusion 13P, and the stacked light-emitting structure layer, consisting of the protrusion 13P, the active layer 15, the electron barrier layer 17, and the p-type semiconductor layer 18, is formed as a cylindrical mesa with a central axis CZ. As shown in Figures 1 and 2A, the p-type semiconductor layer 18 has a current injection portion 18C (current constriction portion) whose cross-section perpendicular to the central axis CZ is an annular protrusion. That is, the central axis of the current injection portion 18C is perpendicular to the active layer 15. In this specification, "cylinder" includes "elliptical cylinder".

[0018] The current injection section 18C has an annular shape coaxial with the central axis CZ of the protrusion 13P of the n-type semiconductor layer 13, and an insulating film 21 is formed on the inner and outer circumferences of the current injection section 18C. The surfaces of the current injection section 18C and the insulating film 21 are formed as substantially the same plane (planar).

[0019] The current injected into the vertical-cavity surface-emitting laser 10 flows through the current injection section 18C. The region inside the current injection section 18C is the resonance region.

[0020] Figure 2B is a schematic plan view of the p-type semiconductor layer 18 and the current injection section 18C as seen from above (top view). The current injection section 18C is coaxial with the central axis CZ and has an annular shape with an inner diameter of DI, an outer diameter of DO, and a width of W.

[0021] The vertical-cavity surface-emitting laser 10 of this embodiment has a chip size of 300 μm square, and the current injection section 18C has an inner diameter DI = 8 μm, an outer diameter DO = 12 μm, and a width W = 2 μm.

[0022] The inner circumference radius RI (=DI / 2) of the current injection section 18C is greater than half the radius R of the p-type semiconductor layer 18 (=R / 2) (R / 2 < RI). Furthermore, the width W of the current injection section 18C is preferably between 1 / 4 and 1 / 2 of the inner circumference radius RI (RI / 4 ≤ W ≤ RI / 2), and more preferably between 1 / 8 and 1 / 4 of the inner diameter DI of the current injection section 18C (RI / 8 ≤ W ≤ RI / 4).

[0023] In other words, because the current injection section 18C (current injection section) is annular, the temperature in the center of the resonator is high and the temperature near the inner circumference of the current injection section 18C is low. However, because it has a large diameter and a narrow ring width, the temperature uniformity of the resonator region is improved and the temperature rise is suppressed.

[0024] More specifically, the inner diameter DI of the current injection section 18C is preferably 6 μm or more, the outer diameter DO is preferably 9 μm or more, and the width W is preferably 1.5 μm or more and 3 μm or less.

[0025] On the other hand, if the width W is less than RI / 4, optical loss (leakage light) occurs in the resonator, and if the width W exceeds RI / 2, the uniformity of the temperature distribution inside the resonator may be impaired.

[0026] A transparent conductive film 22 is formed on the current injection section 18C and the insulating film 21. A circular spacer layer 24 is formed on the transparent conductive film 22, and a dielectric DBR 25 (second reflector) is provided on the spacer layer 24. The spacer layer 24 acts as a phase adjustment layer. The spacer layer 24 and the dielectric DBR 25 have a cylindrical shape coaxial with the central axis CZ. Alternatively, the dielectric DBR 25 may be formed on the transparent conductive film 22 without forming the spacer layer 24.

[0027] Furthermore, an annular n-electrode 27 electrically connected to the n-type semiconductor layer 13 is provided on the flat exposed surface at the bottom of the outer protrusion 13P where the n-type semiconductor layer 13 is exposed. Additionally, an annular p-electrode 28 electrically connected to the transparent conductive film 22 is provided on the transparent conductive film 22 outside the spacer layer 24.

[0028] The semiconductor DBR12 (first reflector) and the dielectric DBR25 (second reflector) constitute a resonator, and the light emitted from the active layer 15 is emitted through the AR coating 29 on the back surface of the substrate 11 (emitted light LE).

[0029] The thicknesses of the n-type semiconductor layer 13 (first semiconductor layer) and the p-type semiconductor layer 18 (second semiconductor layer) can be appropriately adjusted according to the design values ​​of the position of the active layer 15 and insulating film 21 (current constriction layer) and the resonator length relative to the intensity distribution of standing waves within the resonator. The distance between the first reflector 12 and the second reflector 25 is preferably a natural number multiple of the wavelength λ within the medium (n × λ, where n is a natural number).

[0030] To efficiently confine light in the light emission (vertical) direction, it is preferable to design the active layer 15 to be located in the region corresponding to the antinode of the standing wave intensity distribution.

[0031] Furthermore, it is preferable that at least one antinode of a standing wave is located within the final barrier layer 15L. From this perspective, it is preferable that the thickness of the final barrier layer 15L is 40 nm or more.

[0032] Figure 3 schematically shows the band structure of the conduction band of the vertical cavity surface-emitting laser 10. Note that the band structure from the n-type semiconductor layer 13 to the p-type semiconductor layer 18 is shown.

[0033] The active layer 15 is composed of a multiple quantum well layer 15Q and a final barrier layer 15L formed on the final well layer 15W of the multiple quantum well layer 15Q. The multiple quantum well layer 15Q has first to n-th well layers (n is an integer of 3 or more). The multiple quantum well layer 15Q of the present embodiment is composed of four well layers 15W of QW1 to QW4, which are first to fourth quantum well layers (hereinafter also simply referred to as well layers), and barrier layers 15B provided therebetween. The final well layer 15W (QW4) is the well layer adjacent to the final barrier layer 15L.

[0034] The barrier layer 15B and the final barrier layer 15L (intermediate layer) have the same crystal composition. The final barrier layer 15L is a layer between the final well layer 15W and the electron barrier layer 17. The final barrier layer 15L is a layer thicker than the barrier layer 15B, which has a function of preventing electrons contained in the multiple quantum well layer 15Q from leaking out and preventing impurities (for example, impurities in the semiconductor layer such as intrusion of excess In in the crystal growth atmosphere) from entering the multiple quantum well layer 15Q.

[0035] Note that the final barrier layer 15L may have a layer with the same composition as the barrier layer 15B as the final barrier layer 15B, and a layer with a composition different from that of the final barrier layer 15B between itself and the electron barrier layer 17.

[0036] The final barrier layer 15L has a layer thickness ti, the electron barrier layer 17 formed on the final barrier layer 15L has a layer thickness ta, and the p-type semiconductor layer 18 has a layer thickness tb. The electron barrier layer 17 has a first electron barrier layer 17A and a second electron barrier layer 17B.

[0037] (2) Method for Manufacturing Vertical Cavity Surface Emitting Laser Hereinafter, a method for manufacturing the vertical cavity surface emitting laser 10 will be described with reference to the manufacturing flowchart shown in FIG. 4.

[0038] (Step S11) Crystal growth of a semiconductor laminated structure was performed by a metal organic vapor phase epitaxy (MOVPE) method. First, a base layer 11B made of GaN with a layer thickness of about 1 μm was grown on the substrate 11. The substrate 11 is a GaN substrate, which is a C-plane GaN substrate inclined by 0.5° from the C-plane toward the M-plane direction and by 0±0.1° toward the A-plane direction.

[0039] Note that although the description is given by taking the case of manufacturing using the MOVPE method as an example, the manufacturing can also be performed by other known crystal growth methods such as the molecular beam epitaxy (MBE) method.

[0040] Subsequently, a semiconductor DBR 12 which is a distributed Bragg reflector was formed on the underlayer 11B (GaN layer). The semiconductor DBR 12 (first reflector) was formed by stacking 42 pairs of an n-type GaN film and an AlInN film. Each semiconductor film of the semiconductor DBR 12 has a film thickness of λ / 4n (n is the refractive index of each semiconductor film) with respect to the emission wavelength λ of the active layer 15.

[0041] (Step S12) An n-type semiconductor layer 13 (first semiconductor layer), which is an n-type GaN layer doped with Si (silicon), was grown on the semiconductor DBR 12 to a layer thickness of 350 nm.

[0042] (Step S13) Subsequently, barrier layers 15B (barrier layers) and well layers 15W (quantum well layers) were alternately formed on the n-type semiconductor layer 13, to form a multiple quantum well layer 15Q having four well layers 15W. A final barrier layer 15L was grown on the final well layer 15W. Note that nitrogen (N was used as the carrier gas ２ ) alone.

[0043] The crystal growth of the four well layers 15W was performed under the same growth conditions, that is, with the same growth temperature, source gas phase ratio, growth time, and the like. Further, crystal growth was also performed for each of the barrier layers 15B under the same growth conditions. The crystal growth of the four well layers 15W was performed such that the EL peak wavelength (emission peak wavelength of the active layer 15) exists on the shorter wavelength side relative to the resonance wavelength, and the difference therebetween, that is, the detuning amount DT, is equal to or greater than a predetermined value. Here, the crystal growth was performed such that the detuning amount DT was 6 nm.

[0044] Specifically, the well layer 15W is made of GaInN (layer thickness: 3 nm), and the barrier layer 15B is made of GaN (layer thickness: 4 nm). The compositions and layer thicknesses of the barrier layer 15B and the four well layers 15W can be appropriately selected according to the desired emission wavelength, emission characteristics, and the like.

[0045] In this specification, the four well layers 15W will be described in order of growth as the first well layer QW1, the second well layer QW2, the third well layer Q3, and the fourth well layer QW4 (final well layer).

[0046] (Step S14) Next, undoped GaN was grown as the final barrier layer 15L with a thickness of 120 nm on the final well layer 15W (QW4) of the multiple quantum well layer 15Q, under the same growth conditions as the barrier layer 15B.

[0047] More specifically, a thin GaN layer (not shown) was first grown under the same growth conditions as the barrier layer 15B. This GaN layer functions as a cap layer and has a thickness of 1-10 nm. While it is preferable to grow this cap layer, it is not required.

[0048] (Step S15) Next, an electron barrier layer 17 (EBL layer) made of Mg (magnesium) doped AlGaN and having a total layer thickness of 10 nm was grown. The electron barrier layer 17 was formed as a two-stage electron barrier layer, as shown in Figure 3. More specifically, the electron barrier layer 17 consists of a first electron barrier layer 17A made of Mg-doped AlGaN (Al composition: 0.42) with a layer thickness of 6 nm, and a second electron barrier layer 17B made of Mg-doped AlGaN (Al composition: 0.30) with a layer thickness of 4 nm.

[0049] Although the example given illustrates the case where the electron barrier layer 17 consists of multiple electron barrier layers, it may also be a single layer. For example, the electron barrier layer 17 may consist of a single layer of Mg-doped AlGaN (Al composition: 0.30) and have a layer thickness of 10 nm. Alternatively, the electron barrier layer 17 may be a composition gradient layer in which the Al composition changes in the stacking direction.

[0050] (Step S16) Next, a p-GaN layer was grown on the electron barrier layer 17 (p-AlGaN) as a p-type semiconductor layer 18 (second semiconductor layer) with a wavelength of 83 nm.

[0051] As described above, the wafer was grown, and its outer periphery was etched to reach the interior of the n-type semiconductor layer 13 to form a cylindrical mesa-shaped stacked semiconductor structure.

[0052] (Step S17) Next, the p-type semiconductor layer 18, which is the uppermost semiconductor layer of the mesa structure, was etched by dry etching to a depth of approximately 20 nm, to form a current injection portion 18C that is an annular convex portion coaxial with the central axis CZ.

[0053] In the inner and outer recesses of the current injection portion 18C formed by etching, SiO for lateral current and optical confinement ２ was deposited to a thickness of 20 nm, so that the surfaces of the current injection portion 18C and the insulating film 21 were formed as a substantially flat surface. As a result, the recesses of the p-type semiconductor layer 18 were planarized, and a current confinement structure composed of the current injection portion 18C and the insulating film 21 was formed.

[0054] The annular convex portion (central axis: CZ) of the p-type semiconductor layer 18 functions as the current injection portion 18C. Here, the insulating film 21 may be formed so as to extend to the side surface of the cylindrical mesa-shaped semiconductor structural layer or to the upper surface of the n-type semiconductor layer 13 (not shown).

[0055] (Step S18) Next, an ITO (indium tin oxide) film was deposited to a thickness of 20 nm as the transparent conductive film 22 on the p-type semiconductor layer 18 and the insulating film 21. Subsequently, as the spacer layer 24, a dielectric (Nb ２ O ５ ) was deposited to a thickness of 38 nm. The spacer layer 24 functions as a phase adjustment layer.

[0056] Further, on the spacer layer 24, a dielectric DBR 25 (second reflector) was deposited. The dielectric DBR 25 is composed of SiO ２ (11 layers) and Nb ２ O ５ (10 layers) forming 10.5 pairs. Note that the dielectric DBR 25 is preferably formed so as to be coaxial with the cylindrical mesa of the p-type semiconductor layer 18.

[0057] (Step S19) Next, an n-electrode 27 was formed on the bottom of the outer peripheral portion of the n-type semiconductor layer 13, and a p-electrode 28 was formed on the transparent conductive film 22. Further, the back surface of the substrate 11 was polished, and Nb ２ O ５ / SiO ２An AR (anti-reflective) coating 29 consisting of two layers was formed. With this, the formation of the vertical cavity type surface-emitting laser 10 was completed.

[0058] Furthermore, the final barrier layer 15L described above is not limited to GaN. That is, although the final barrier layer 15L was described as a GaN layer, nitride semiconductor layers of other compositions, such as InGaN, AlGaN, InAlGaN, etc., may also be used. In addition, although the final barrier layer 15L was described as an undoped layer, it may contain dopants diffused from the electron barrier layer 17 or the p-type semiconductor layer 18.

[0059] Furthermore, the composition and thickness of the electron barrier layer 17 are merely illustrative. The electron barrier layer 17 may have a thickness of, for example, 3 to 30 nm, and the Al composition may be adjusted within the range of 10 to 70%.

[0060] Although the electron barrier layer 17 was described as a p-type semiconductor layer (p-AlGaN), it may also be a p-type semiconductor layer that was grown as an i-layer and contains dopants diffused from the p-type semiconductor layer 18.

[0061] Furthermore, although the example given illustrates a case where the active layer 15 has four well layers 15W, it is sufficient for it to have three or more well layers.

[0062] Furthermore, the p-type semiconductor layer 18 may be composed of multiple semiconductor layers, including layers with different compositions and / or doping concentrations, and an undoped layer. Similarly, the n-type semiconductor layer 13 may also be composed of multiple semiconductor layers, including an undoped layer.

[0063] The dielectric DBR25 is SiO ２ Membrane and Nb ２ O ５ Although the example given illustrates the case where the material consists of films, it may also be composed of dielectric films with different refractive indices in other combinations. Alternatively, it may be composed of a semiconductor DBR made up of semiconductor films with different refractive indices.

[0064] (3) Multiple quantum well active layer and peak gain wavelength In the vertical cavity type surface-emitting laser 10 described above, the active layer 15 has four well layers 15W (QW1 to QW4).

[0065] Figure 5 schematically shows the gain profiles of each well layer of the active layer 15 in this embodiment. More specifically, it schematically shows the gain profiles (dashed lines) and combined gain profile GA of each well layer (QW1 to QW4) of the active layer 15, which has four well layers (QW1 to QW4) grown under the same growth conditions, and all barrier layers, including the final barrier layer 15B, grown under the same growth conditions.

[0066] Even when all well layers of a multiple quantum well active layer are grown under the same growth conditions, and all barrier layers are grown under the same growth conditions, i.e., when the growth temperature, raw material gas phase ratio, growth time, etc., are the same, the peak gain wavelengths will not be the same when current is driven, as shown in Figure 5, and the gain profiles of each well layer (QW1 to QW4) will be separated.

[0067] This is due to a growth mechanism called CPE (Compositional Pulling Effect) when growing quantum well layers (see Non-Patent Documents 1 and 2). More specifically, the amount of incorporation of atoms (In: indium) changes depending on the amount of strain on the crystal surface during growth.

[0068] Furthermore, according to the analysis results of the active layer 15 of this embodiment using EDX (energy dispersive X-ray spectrometer), the well layers QW1 to QW4 (Ga １－Z In Z The In composition (Z1 to Z4) of N) was Z1 < Z2 < Z3 < Z4. In other words, it was confirmed that the In composition (Z) of the well layers increased in the order of growth, i.e., in the order of stacking. That is, when the In composition of the k-th well layer (k = 1, 2, ..., n) is Zk, then Z1 < Z2 < ... < Zn, and the In composition of the first to n-th well layers is larger for well layers closer to the p-type semiconductor layer 18 (second semiconductor layer).

[0069] Furthermore, since the amount of strain differs in each quantum well layer, the band structure deforms due to the piezoelectric effect. The amount of strain (compressive stress) ε1 to ε4 in each well layer (QW1 to QW4) increases as it approaches the final barrier layer 15L. That is, ε1 < ε2 < ε3 < ε4.

[0070] The increase in compressive strain due to the increasing In composition and the lengthening of the gain wavelength based on the accumulated compressive strain are not only due to the simple modulation of the In composition described above, but also largely due to the fact that the piezoelectric field of each well layer increases as the amount of strain increases. Furthermore, because the effective mass of electrons and holes is larger compared to other material systems, a spatial distribution occurs in the carrier density during operation.

[0071] These three main factors occur in a complex manner, resulting in the phenomenon where the peak gain wavelength differs for each quantum well layer.

[0072] In a growth method in which crystal growth is performed on the well layers (quantum well layers) and barrier layers of a multiple quantum well active layer under the same growth conditions, the peak gain wavelength (dashed line) increases in the order of well layers QW1 to QW4, i.e., in the order of growth, as schematically shown in Figure 5. Therefore, the composite gain profile GA is broad, and it can be seen that the gain gradually increases from the gain peak of QW1 to the gain peak of QW4. The interval between the peak gain wavelengths of each well layer was approximately 3.6 nm.

[0073] Figure 6 schematically shows the gain and combined gain profiles GA of the well layers QW1 to QW4 shown in Figure 5, and the EL (Electroluminescence) intensity profile of the active layer 15 at the threshold current.

[0074] Here, the resonant wavelength λc is the wavelength set as the wavelength of the resonator of the VCSEL element. Specifically, it corresponds to the wavelength of the light emitted as laser light in the resonator formed by the semiconductor DBR12 (first reflector) and the dielectric DBR25 (second reflector). For example, the resonant wavelength λc at room temperature is set to 440 nm.

[0075] The resonant wavelength λc and the combined gain profile GA of the active layer 15 are temperature-dependent. Furthermore, the EL profile changes with the temperature of the combined gain profile GA. Specifically, the resonant wavelength λc has a temperature dependence (rate of change) RW = Δλc / ΔT (where T is the junction temperature), and the combined gain profile GA has a temperature change rate RG = Δλp(EL) / ΔT. The factors causing thermal saturation (saturation of optical output) during high-temperature operation are explained below. Here, λp represents the EL peak wavelength.

[0076] The resonant wavelength λc is lengthened by thermal expansion, and its rate of change at temperature RW = Δλc / ΔT is, for example, 0.012 nm / K. On the other hand, the combined gain profile GA is shifted to the longer wavelength side by heat, and its rate of change at temperature RG is, for example, 0.07 nm / K. Therefore, the combined gain profile GA is larger than the rate of change at temperature RW of the resonant wavelength λc (RW < RG). Also, the EL profile is shifted to the longer wavelength side by heat, and its rate of change at temperature is approximately equal to the rate of change at temperature RG of the combined gain profile GA.

[0077] In other words, if the resonant wavelength at room temperature is λcr, the EL peak wavelength at room temperature is λpr(EL), the junction temperature of the VCSEL element is T, and the temperature rise from room temperature is ΔT, then the resonant wavelength λc and the EL peak wavelength λp(EL) are expressed by the following equations, respectively.

[0078] λc = λcr + Δλc = λcr + 0.012 nm × ΔT λp(EL) = λpr(EL) + Δλp = λcr + 0.07 nm × ΔT Furthermore, as schematically shown in Figure 6, the active layer 15 has a gain wavelength band GB (width: Δλact = λact1 - λact2) in which the gain of the active layer 15 exceeds the gain threshold Gth, and the laser oscillates in the temperature range in which the resonant wavelength λc exists within the gain wavelength band GB (Δλact).

[0079] In other words, as the drive current increases and the temperature of the active layer 15 rises, the gain wavelength band GB itself shifts to a longer wavelength by 0.07 nm × ΔT with respect to the temperature increase ΔT, and overtakes the resonant wavelength λc at high temperatures. When λact2 overtakes the resonant wavelength λc, optical output saturation occurs, and the optical output drops significantly.

[0080] Therefore, by widening the width Δact of the gain wavelength band GB, it is possible to operate at higher temperatures. Consequently, oscillation can be maintained up to higher drive currents, thus maintaining a high output state. In other words, by widening the width Δλact of the gain wavelength band GB, and ensuring that the resonant wavelength λc exists within the width Δλact where gain can be obtained even at high temperatures, the oscillation state at high temperatures can be maintained.

[0081] Figure 7 schematically shows the combined gain profile GA and EL profile of the active layer 15 at room temperature. The resonance wavelength λcr and EL peak wavelength λpr(EL) at room temperature are also shown.

[0082] At room temperature, the EL peak wavelength λpr(EL) is set to the shorter wavelength side relative to the resonant wavelength λcr, and the difference is defined as the detuning amount DT. In this embodiment, the detuning amount DT of the vertical cavity surface-emitting laser 10 was 8 nm.

[0083] After careful consideration, it has been found that the detuning amount DT = λcr - λpr (EL) is preferably 4 nm or more, and more preferably 6 nm or more. This allows oscillation to be maintained up to higher drive currents, thus enabling the maintenance of a high-power state. Consequently, a vertical-cavity surface-emitting laser with excellent high-temperature characteristics and high-power characteristics can be obtained. However, if the detuning amount DT is too large, the oscillation threshold at room temperature rises, so it is preferably 12 nm or less.

[0084] As the injection current into the active layer 15 increases, the heat generated in the active layer 15 increases, and the combined gain profile GA and EL profile shift to the longer wavelength side. However, the gain on the shorter wavelength side can be utilized even during high-power operation. Therefore, the gain is maintained even during high-power operation, and a vertical-cavity light-emitting element with excellent high-temperature characteristics and high-power characteristics is realized.

[0085] Figure 8 schematically shows the gain of well layers QW1 to QW4 and the combined gain profile GA in the vertical-cavity surface-emitting laser of Comparative Example CX1. The vertical-cavity surface-emitting laser of Comparative Example CX1 differs from the vertical-cavity surface-emitting laser 10 of this embodiment in the configuration of well layers QW1 to QW4. Specifically, the gain peak wavelength of QW4 is between the gain peak wavelengths of QW2 and QW3. Also, as described above, the vertical-cavity surface-emitting laser 10 of this embodiment has an annular current injection section 18C in the p-type semiconductor layer 18, but the vertical-cavity surface-emitting laser of Comparative Example CX1 differs in that it has a circular current injection section coaxial with the central axis CZ of 5 μmΦ. The other configurations are the same as the vertical-cavity surface-emitting laser 10 of this embodiment.

[0086] In Comparative Example CX1, by reducing the In composition of the final well layer QW4, the gain peak wavelength of the final well layer QW4 is set to a wavelength Δλ = 3.6 nm shorter than the gain peak wavelength of the third well layer QW3, resulting in a narrower wavelength bandwidth of the composite gain profile GA. In Comparative Example CX1, because the wavelength bandwidth in which gain can be obtained is narrow, the effect of redshift is large, and the optical output is prone to saturation.

[0087] Figure 9 shows the optical output-current characteristics (L-I characteristics) of the vertical-cavity surface-emitting laser 10 of this embodiment, and Figure 10 shows the optical output-current characteristics (L-I characteristics) of the vertical-cavity surface-emitting laser of Comparative Example CX1.

[0088] As shown in Figure 10, the vertical-cavity surface-emitting laser of Comparative Example CX1 had a maximum optical output of 5.6 mW and an optical saturation current of 17 mA.

[0089] As shown in Figure 9, in the vertical-cavity surface-emitting laser 10 of this embodiment, the maximum optical output was 29.5 mW and the optical saturation current was 56 mA. In the vertical-cavity surface-emitting laser 10, an unprecedentedly large optical output was obtained, and the drive current was also significantly increased. Furthermore, at the point of optical saturation current, while the junction temperature was 170°C in the comparative example, it was confirmed that the optical output increased to a junction temperature of 215°C in the present invention.

[0090] Therefore, according to this disclosure, it is possible to provide a vertical resonator type light-emitting element and a method for manufacturing a vertical resonator type light-emitting element having extremely excellent high-temperature characteristics and high-power characteristics.

[0091] [Second Embodiment] Figure 11A is a schematic cross-sectional view showing the structure of a vertical-cavity surface-emitting laser 50 (VCSEL) according to a second embodiment of the present invention. Figure 11B is a schematic view of the vertical-cavity surface-emitting laser 10 as seen from above.

[0092] The vertical-cavity surface-emitting laser 50 has a stacked light-emitting structure layer consisting of a protrusion 13P of an n-type semiconductor layer 13, an active layer 15, an electron barrier layer 17, and a p-type semiconductor layer 58. This stacked light-emitting structure layer is formed as a cylindrical mesa having a central axis CZ.

[0093] In this embodiment, the vertical-cavity surface-emitting laser 50 has a circular current injection section 58C provided in the p-type semiconductor layer 58, which is coaxial with the central axis CZ, instead of the annular current injection section 18C of the vertical-cavity surface-emitting laser 10 of the first embodiment. The other configurations are the same as those of the vertical-cavity surface-emitting laser 10.

[0094] More specifically, the p-type semiconductor layer 58 has a current injection portion 58C, which is a convex portion with a circular cross-section perpendicular to the central axis CZ. An insulating film 51 is formed on the outer periphery of the current injection portion 58C. The surfaces of the current injection portion 58C and the insulating film 51 are formed as substantially flat surfaces.

[0095] Furthermore, the vertical-cavity surface-emitting laser 50 had substantially the same active layer 15 as the vertical-cavity surface-emitting laser 10 of the first embodiment, and at room temperature, the detuning amount DT of the EL peak wavelength with respect to the resonance wavelength was 8 nm.

[0096] Figure 12 shows the optical power-current characteristics (L-I characteristics) of the vertical-cavity surface-emitting laser 50 of the second embodiment. In the vertical-cavity surface-emitting laser 50, the maximum optical power was 13.8 mW and the optical saturation current was 20 mA.

[0097] In the vertical-cavity surface-emitting laser 50, it was confirmed that the drive current could be greatly increased, resulting in a large optical output.

[0098] [Variations]

[0099] (1) Modification 1 Figure 13 shows the band structure of the conduction band of the active layer 65 in Modification 1 of the vertical cavity surface-emitting laser of the above embodiment.

[0100] The active layer 65 has a multiple quantum well layer 65Q consisting of first to nth well layers (where n is an integer of 3 or more). More specifically, the active layer 65 consists of a multiple quantum well layer 65Q consisting of four quantum well layers and a final barrier layer 15L formed on the final well layer 65W4 of the multiple quantum well layer 65Q.

[0101] The multiple quantum well layer 65Q has first to fourth well layers 65W1 to 65W4 (QW1 to QW4) whose thickness increases as they approach the p-type semiconductor layer 18. When the multiple quantum well layer 15Q has first to n well layers, if the thickness of the k-th well layer is Tk (k = 1, 2, ..., n), then T1 < T2 < ... < Tn, and the thickness of the well layers is greater for well layers closer to the p-type semiconductor layer 18 (second semiconductor layer).

[0102] The layer thicknesses of the first to fourth well layers 65W1 to 65W4 are, for example, 3.0 nm, 3.1 nm, 3.2 nm, and 3.4 nm, respectively. However, the layer thickness of each quantum well layer can be selected as appropriate. Also, the number of quantum well layers is not limited to four and can be selected as appropriate, but it is preferable to have three or more.

[0103] As the thickness of the quantum well layer increases, the inter-quantum energy (band gap) of the quantum well layer decreases, and the gain of the quantum well layer becomes longer wavelength. Therefore, the gain wavelength bandwidth Δλact of the composite gain profile GA of the multiple quantum well layer 65Q can be expanded. By expanding the gain wavelength bandwidth Δλact, gain can be obtained at higher temperatures, allowing oscillation to be maintained even with higher drive currents, and thus maintaining a high-power state.

[0104] (2) Modification 2 Figure 14 is a schematic cross-sectional view showing the structure of a vertical cavity surface-emitting laser 70 (VCSEL), which is a modification 2 of the above-described embodiment.

[0105] The vertical-cavity surface-emitting laser 70 of the modified example 2 is provided with a conductive semiconductor DBR 72 (first reflector). For example, the conductive semiconductor DBR 72 is formed by repeating four layers 40 times: a first layer which is an n-AlInN layer, a second layer which is a GaN cap layer, a third layer which is an n-AlGaN composition gradient layer, and a fourth layer which is an n-GaN layer.

[0106] Furthermore, the fact that an annular current injection section 18C is provided in the p-type semiconductor layer 18 is the same as in the vertical-cavity surface-emitting laser 10 of the first embodiment. The entire stacked semiconductor layer of the vertical-cavity surface-emitting laser 10 has a cylindrical shape, but it may also have a prismatic shape.

[0107] Furthermore, a circular n-electrode 77 having an opening coaxial with the central axis CZ is provided on the back surface of the substrate 11. This opening functions as a light-emitting section 77E. The light-emitting section 77E has a larger diameter than the outermost edge of the current-injecting section 18C.

[0108] Furthermore, according to the structure of the modified example 2, by providing the n electrode on the back surface of the substrate 11, the current flows more easily through the resonator almost vertically, and the oscillation mode becomes more stable.

[0109] Therefore, the vertical-cavity surface-emitting laser 70 of the modified example 2 has high heat dissipation, and the oscillation mode remains stable while maintaining a high output state even at higher temperatures and drive currents.

[0110] [Third Embodiment] Figure 15A is a schematic cross-sectional view showing the structure of a two-dimensional array type vertical cavity surface-emitting laser array 80 (VCSEL array) according to the third embodiment of the present invention. Figure 15B is a schematic plan view showing the back surface of the vertical cavity surface-emitting laser array 80. Note that Figure 15A shows a cross-section along the line A-A shown in Figure 15B.

[0111] The vertical-cavity surface-emitting laser array 80 has a structure in which a plurality of the vertical-cavity surface-emitting lasers 70 of the modified example 2 described above are arranged, for example, in a triangular lattice.

[0112] Specifically, as shown in Figure 15A, a multilayer semiconductor layer on which multiple vertical-cavity surface-emitting lasers 70 of Modification 2 are formed is located on the opposite side of the substrate 11, i.e., a p-type semiconductor layer 18 (the side of the second semiconductor layer) is etched with grooves 11G extending to the interior of the substrate 11, thereby forming multiple vertical-cavity surface-emitting lasers 70. The sides of the vertical-cavity surface-emitting lasers 70 are protected by an insulating film 71. The insulating film 71 is formed to cover the inner wall and bottom surfaces of the grooves 11G.

[0113] More specifically, the stacked semiconductor layer is etched from the p-type semiconductor layer 18 to the interior of the substrate 11, forming a vertical-cavity surface-emitting laser array 80 in which multiple cylindrical vertical-cavity surface-emitting lasers 70 are arranged. In other words, the multiple vertical-cavity surface-emitting lasers 70 are formed as a single unit, sharing the substrate 11.

[0114] Furthermore, a common n-electrode 82 for the multiple vertical-cavity surface-emitting lasers 70 is provided on the back surface of the substrate 11. The n-electrode 82 has an aperture corresponding to the light-emitting portion 77E of each of the multiple vertical-cavity surface-emitting lasers 70. The light-emitting portion 77E has a circular shape coaxial with the central axis of the vertical-cavity surface-emitting laser 70.

[0115] Preferably, the multiple vertical-cavity surface-emitting lasers 70 are separated by grooves 11G with a depth of at least half the thickness of the substrate 11. In this case, even if the vertical-cavity surface-emitting lasers 70 are arranged in an array, it becomes less likely for the current density distribution in the resonator region to be uneven. Also, if the current injection section is formed in an annular shape, the current tends to be unevenly distributed towards the outer periphery rather than in the resonator region, and the current tends to flow through the substrate in a direction parallel to the substrate (i.e., in the direction of adjacent vertical-cavity surface-emitting lasers). Therefore, by making the depth of the grooves 11G at least half the thickness of the substrate 11, each of the multiple vertical-cavity surface-emitting lasers 70 can be driven with a more uniform current.

[0116] Although the example given shows multiple vertical-cavity surface-emitting lasers 70 arranged in a triangular grid, they may also be arranged in a regular grid such as a square grid, or in an arrangement that corresponds to a desired light distribution pattern.

[0117] As described in detail above, this disclosure makes it possible to provide a vertical resonator type light-emitting element and a vertical resonator type light-emitting element array having excellent high-temperature characteristics and high-power characteristics.

[0118] 10, 50, 70: Vertical cavity surface-emitting laser 11: Substrate 11G: Groove 12: First reflector 13: First semiconductor layer (n-type semiconductor layer) 13P: Protrusion 15, 65: Active layer 15B, 65B: Barrier layer 15L: Final barrier layer 15Q, 65Q: Multiple quantum well layer 15W: Well layer 17: Electron barrier layer 18, 58: Second semiconductor layer (p-type semiconductor layer) 18C, 58C: Current injection section 21, 51: Insulating film 22: Transparent conductive film 24: Spacer layer 25: Second reflector 27: n-electrode 28: p-electrode 29: AR coating 65W1 to 65W4: First to fourth well layers 77E: Light emission section 82: n electrode

Claims

1. A vertical resonator type light-emitting element comprising: a first reflector; a first semiconductor layer formed on the first reflector; an active layer comprising a multiple quantum well layer including first to n well layers (n is an integer of 3 or more) sequentially formed on the first semiconductor layer and a final barrier layer formed on the nth well layer which is the final well layer of the multiple quantum well layer; an electron barrier layer formed on the final barrier layer; a second semiconductor layer formed on the electron barrier layer; and a second reflector formed on the second semiconductor layer, wherein the first to n well layers have an In composition, the In composition of the first to n well layers is greater for well layers closer to the second semiconductor layer, the EL peak wavelength of the multiple quantum well layer is shorter than the resonance wavelength of the resonator formed by the first reflector and the second reflector, and the detuning amount, which is the difference between the EL peak wavelength and the resonance wavelength, is 4 nm or more.

2. The vertical resonator type light-emitting element according to claim 1, wherein the thickness of the first to nth well layers is greater for well layers closer to the second semiconductor layer.

3. The vertical resonator type light-emitting element according to claim 1, wherein the second semiconductor layer has an annular current injection portion whose central axis is perpendicular to the active layer.

4. The vertical resonator type light-emitting element according to claim 3, wherein the stacked semiconductor layer comprising the active layer, the electron barrier layer, and the second semiconductor layer has a cylindrical shape, the current injection portion is coaxial with the stacked semiconductor layer, and when the radius of the inner circumference of the current injection portion is RI and the radius of the second semiconductor layer is R, R / 2 < RI.

5. The vertical resonator type light-emitting element according to claim 3, wherein the multilayer semiconductor layer comprising the active layer, the electron barrier layer, and the second semiconductor layer has a cylindrical shape, the current injection portion is coaxial with the multilayer semiconductor layer, and when the width of the current injection portion is W and the inner diameter of the current injection portion is DI, RI / 8 ≤ W ≤ RI / 4 is satisfied.

6. The vertical resonator type light-emitting element according to claim 1, wherein the second reflector is a conductive semiconductor DBR.

7. The vertical resonator type light-emitting element according to claim 1, wherein the thickness of the final barrier layer is 40 nm or more.

8. The vertical resonator type light-emitting element according to claim 1, wherein the electron barrier layer comprises aluminum (Al), and the electron barrier layer comprises a first electron barrier layer and a second electron barrier layer having a lower Al composition than the first electron barrier layer.

9. The vertical resonator type light-emitting element according to claim 8, wherein the first electron barrier layer is thicker than the second barrier layer.

10. The vertical resonator type light-emitting element according to any one of claims 1 to 9, wherein the vertical resonator type light-emitting element is a nitride surface-emitting laser made of a gallium nitride (GaN) semiconductor.

11. A vertical resonator type light-emitting array having a plurality of vertical resonator type light-emitting elements according to any one of claims 1 to 9, wherein the plurality of vertical resonator type light-emitting elements are separated by grooves extending from the side of the second semiconductor layer to the interior of the substrate and arranged with the substrate as a common element.

12. The vertical resonator type light-emitting array according to claim 11, wherein the groove extending into the interior of the substrate has a depth of at least half the thickness of the substrate.

13. The vertical resonator type light-emitting array according to claim 11, wherein the second reflector of each of the plurality of vertical resonator type light-emitting elements is a conductive semiconductor DBR, and an electrode common to the plurality of vertical resonator type light-emitting elements is provided on the back surface of the substrate.

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